Switching device for electronic musical instrument with sustain effect



United States Patent Inventor Yuichi Teranishi Akishima-shi, Tokyo,Japan Appl. No. 734,764 Filed June 5, 1968 Patented Oct. 27, 1970Assignee Hitachi,Ltd.

Tokyo, Japan a corporation of Japan. Priority June 19, 1967 Japan42/38,849

U.S. Cl.... 84/126 Int. Cl GlOh I/02 Field of Search 84/ l .04,

Primary ExaminerWarren E. Ray Assistant Examiner-Stanley .I. WitkowskiAttorney-Craig, Antonelli, Stewart & Hill ABSTRACT: A switching devicefor electronic musical instrument, comprising a transistor, a CRtime-constant circuit for producing a sustain effect, and a PN junctiondiode connected with the base electrode of said transistor in the sameconducting direction as the emitter-base junction of said transistor,wherein an input signal is supplied to said base electrode and an outputsignal is taken from the collector electrode of said transistor.

SWITCHING DEVICE FOR ELECTRONIC MUSICAL INSTRUMENT WITII SUSTAIN EFFECTThis invention relates to improvements in a switching device for anelectronic musical instrument.

In general, an electronic musical instrument uses such a system that anumber of signals of different frequencies are suitably selected byactuating keys arranged in a keyboard which are associated with aswitching device, and the signals thus selected are combined or mixedwith each other to produce a desired musical tone. In accordance with aconventional example, the switching device of this type comprises anelectronic amplifying element, a manual switch such as a key switch forcontrolling said electronic amplifying element, and a sustain" circuitor envelope forming circuit for producing a sustained attenuation effect(referred to as sustain" effect) for an output signal. Such device is akind of gate circuit device, as will be described later, which is alsoreferred to as sustain gate circuit. The feature of such switchingdevice is that there is obtained an output signal of which the amplitudeis attenuated not momentarily but gradually by the actuation of saidkeys.

In an electronic musical instrument using the above switching device,however, there is a tendency that a leakage signal occurs at the outputterminal of the switching device when the manual switch is opened orwhen the instrument is not played, thus resulting in noise which can notbe neglected. This has become a serious problem with an electronicmusical instrument.

It is an object of this invention to provide a novel and improvedswitching device for an electronic musical instrument, which is capableof minimizing a leakage signal.

Another object of this invention is to provide a miniaturized switchingdevice using a transistor.

Still another object of this invention is to provide a switching deviceusing a transistor, which is so designed as to prevent the transistorfrom being damaged.

In accordance with the present invention, an unsymmetrical type of anelement such as a PN junction diode (an element of which the forwardvoltage-current characteristic is unsymmetrical with respect to thebackward characteristic) is coupled to the input circuit of an amplifierelement such as a transistor, whereby the above objects can beaccomplished. The amplifier element and unsymmetrical type element areprovided with a first or a second bias condition by biasing means inaccordance with the opening or closing operation of switch means suchfor example as a key switch, respectively. Under the first biascondition, the unsymmetrical type element is biased in the reversedirection, and the amplifier element is so biased that its gain becomessubstantially zero. Under the second bias condition, on the other hand,said unsymmetrical type element is biased in the forward direction, andthe amplifier element is so biased that its gain assumes a predeterminedvalue. Furthermore, a time-constant circuit for delaying the switchingof the bias condition for a'predetermined period of time is combinedwith the biasing means and switch means, whereby said switch means isswitched from on" to of Other objects, features and advantages of thepresent invention will become apparent from the following descriptiontaken in conjunction with the accompanying drawings, in which:

FIG. 1 is a circuit diagram showing a conventional switching device foruse with an electronic musical instrument;

FIG. 2 is a circuit diagram showing the switching device for anelectronic musical instrument according to a first embodiment of thepresent invention; and

FIG. 3 is a circuit diagram showing the switching device for anelectronic musical instrument according to a second embodiment of thepresent invention.

Referring to FIG. 1, there is shown a conventional switching device,wherein the reference numeral 1 represents a tone generator, 2 atransistor. 5 and 6 bias resistors, 7 a load resistor, and 8 an emitterresistor. A circuit 4 is a circuit for producing the sustain effect,which is constituted by a resistor 9 and a capacitor 10. +V represents apower source for operating the transistor 2, and +V, denotes a powersource for affording a reverse bias to the emitter of the transistor.The reference numeral 3 represents a key switch which is closed bydepressing the key, 11 a resistor, and 12 an output terminal. A resistoris usually connected between the tone generator 1 and the node 13,though it is not shown in FIG. 1.

In operation, initially the emitter-base junction of the transistor 2 isdeeply reverse biased by the power source +V with the key switch beingmaintained in the open state. Thus, even if an input signal voltage e,is supplied from the tone generator 1 to the transistor 2, the latter isnormally rendered nonconductive with respect to the input signalvoltage, so that the input signal voltage e, is substantially nottransmitted to the output terminal 12. However, a slight leakage signalappears at the output signal 12. It is presumed that such leakage signalis produced by the fact that the input signal voltage e, is transmittedto the output terminal 12 through the barrier capacitance 14 between thecollector and the base of the transistor 2.

Subsequently, by closing the key switch 3, the emitter-base junction ofthe transistor 2 is biased slightly in the forward direction so that thetransistor is permitted to perform a normal switching operation, thustransmitting the input signal voltage e, to the output terminal 12.

In the foregoing switching device, the transistor is switchinglyrendered nonconductive or conductive by opening or closing the keyswitch 3, thereby performing the switching operation with respect to thesignal from the tone generator, as described above. However, suchconventional device has such a serious drawback that in case it isincorporated in a musical instrument such as an electronic organ whichhas a number of keys, a signal tone from a tone generator is likely tobe heard even when no key is depressed, due to the fact that theopening-closure" ratio (ratio S /S of the level S of an output signalwhich is obtained at the output terminal 12 when the key switch 3 isclosed to the level S of a leakage signal which occurs at the outputterminal 12 when the key switch 3 is opened).

Furthermore. signal e, from the tone generator I is always supplied to anode 13, and the emitter breakdown voltage of the transistor is not toohigh. As a result, there is a tendency that the emitter junction of thetransistor breaks down easily.

The present invention intends to eliminate the above drawbacks of theconventional device. In accordance with an embodiment of the presentinvention, an unsymmetrical element such as a diode is inserted in thebase circuit of a transistor, and there is provided a means for biasingsaid diode in the reverse or forward direction in accordance with theopen or closed state of a switch means such as a key switch. Thus, whenthe key switch is in the open state, the diode represents a highimpedance whereby a signal which is actually supplied to the base of thetransistor is remarkably attenuated. Such signal attenuation contributesto an increase of the opening-closure ratio and reduction of the emitterjunction breakdown trouble.

FIG. 2 shows the device according to a first embodiment of thisinvention, wherein reference numeral 21 represents a tone generator forgenerating a rectangular wave signal, 32 a PN junction diode connectedwith the generator, 22 an NPN transistor to the base electrode of whichis supplied said signal through the diode 32, and 27 a load resistorhaving one end thereof connected with a positive potential source +V andthe other end thereof connected with the collector electrode of thetransistor and an output terminal 33. Reference numeral 24 denotes anenvelope forming circuit consisting of a resistor 28and a capacitor 29and connected with the emitter electrode of the transistor through anemitter resistor 31, and 23 a key switch connected with the emitter ofthe transistor through resistors 30 and 31 for controlling the biasconditions for the transistor and diode. Reference numerals 25 and 26indicate bias resistors for the transistor, and +V., a power source foroperating said envelope forming circuit and biasing the diode 32 inthereverse direction through resistors 25 and 28 when the key switch isopened.

Description will now be made of the operation of the present switchingdevice or sustain gate circuit. First, when the switch 23 is opened, thediode 32 is biased in the reverse direction through the resistors 25 and28 by the potential source +V having a positive potential selected abovethe peak value of the input signal voltage. Capacitor 29 is chargedsubstantially to +V (V), and the potential at the node 35 issubstantially +V (V). The potential +V (V) is selected to be close to orlower than the potential +V, (V). That is, the transistor 22 is sobiased that it may operate in the cut-off region or a low current levelregion in the neighborhood of the cut-off region so that the gainthereof becomes substantially zero. A signal which has been attenuatedby the high impedance of the diode biased in the reverse direction isapplied to the node 34 coupled to the base of the transistor 22, sincethe impedance between the node 34 and the ground potential or referencepotential point is lower than the backward impedance of the diode. Thus,the attenuated signal does not cause the breakdown of the emitterjunction and is substantially prevented from being transmitted to theoutput terminal v33, unlike in the conventional case.

Thereafter, if the switch 23 is closed, then the potential at the node35 becomes equal to the ground potential in a short period of time whichdepends upon the product of the capacitance of the capacitor 29 and theresistance of the resistor 30, since the resistance of the resistor 30is selected to be very low as compared with the resistance of theresistor 28. At this time, the diode is biased in the forward direction,and the operating point for the transistor 22 is so selected as to fallwithin the active region thereof, whereupon the transistor starts theamplifying operation with a predetermined gain so that the signal fromthe tone generator is transmitted to the output terminal 33. The diodewill by no means hinder such signal transmission since it is biased inthe forward direction as described above.

Subsequently, the switch 23 is again returned to the open state. At thistime, the potential at the node is not immediately returned to theinitial value or substantially +V (V). More specifically, the potentialat the node 35 continues to increase gradually in accordance with thecharging characteristic of the capacitor 29 which is substantiallydependent upon the product RC (referred to as time-constant) of theresistance R of the resistor 28 and the capacitance C of the capacitor29. When the potential at the node 35 reaches +V (V), each of the diode32 and transistor 22 will be provided with the initial bias condition sothat the signal will be prevented from being transmitted to the terminal33.

Thus, the transistor 22 will not be provided with the initial biascondition upon the opening of the switch 23. That is, when the switch isopened, the bias condition for the transistor will be gradually changedfrom that which has been imparted thereto with the switch being closedto the steady bias condition by way of a predetermined transient state,so that an output signal will be held at the terminal 33 for apredetermined period of time even if the switch is opened. The amplitudeof the output signal thus held will be substantially exponentiallyattenuated in accordance with the transient characteristic of theforegoing time-constant circuit or envelope forming circuit, so that aso-called envelope is formed in the output signal. In any electronicmusical instrument, such envelope signal is essential to the productionof a natural tone. It is for the purpose of producing such an envelopesignal that use is made of the combination of a key switch with anelectronic amplifier element and an envelope forming circuit rather thana key switch alone, as described above.

The reason for the increase of the opening-closure ratio can beexplained as follows: a sort of high-pass filter is equivalentlyconstituted by the capacitance between the electrodes of the diode 32and the impedance between the node 35 and the ground point, so that thelower order harmonic components of the rectangular wave from thegenerator 21 are cut off by the filter circuit while the higher orderones are permitted to pass therethrough. The higher order harmoniccomponents thus passed are again filtered by a high-pass filtercomprising the collector barrier capacitance and the impedance betweenthe terminal 33 and the ground point, before they reach the outputterminal 33. In this way, only a small amount of extremely high orderharmonic components will appear at the output terminal 33. A signalcontaining such high order harmonic components has not sufficient powerto drive a speaker even if it is amplified in the succeeding circuits.Further, even if the speaker is driven by such a signal, the human earis not sensitive to the high order harmonic components.

As described above, substantially no lower order harmonic components oraudio-frequency components will appear at the node 34, and yet extremelyweak audio-frequency components are further attenuated when they aremade to pass through the collector barrier capacitance. As a result, thelevel S of the leakage signal occurring at the output terminal 33becomes very low, and the opening-closure ratio (S /S is increased.Thus, one of the objects of the present invention can be accomplished.In the foregoing, description has been made of the switching deviceaccording to an embodiment of the present invention, and it has beenfound that such a device can satisfactorily operate when the circuitconstants are selected as indicated in table 1.

TABLE 1 Rectangular signal voltage of the generator 21 (peak value),10V.

Diode 32, 181473.

Transistor 22, NPN silicon transistor 2SC458.

Potential source +V3, 12V.

Potential source 10V.

Resistor 25, 20KS2.

Resistor 26, KSZ.

Resistor 27, 4009.

Resistor 28, 501(8).

Resistor 30, 639.

Resistor 31, 3.2Kt2.

Capacitor 29, 50oF.

, In the circuit arrangement of FIG. 2 which has the circuit iconstantsas indicated in table l, the opening-closure ratio l measured at asignal source frequency of l KHz was in a range lof 80 to dB. The signallevel was measured with respect to i the peak-to-peak value of thevoltage waveform. From this, it will be appreciated that the circuit ofFIG. 2 is advantageous over the circuit of FIG. 1 since with the lattercircuit the opening-closure ratio is about 50 dB at best.-

Referring now to FIG. 3, there is shown the device according to anotherembodiment of this invention, which is different from the device shownin FIG. 2 in that there is provided electronic switching meansconstituted by a common collector transistor circuit. However, these twodevices are substantially similar to each other in respect of the basicoperation. In FIG. 3, the collector electrode of a transistor 42 isgrounded through a load resistor 47, and a signal source 41 is connectedbetween the ground and a diode 52 connected at the node 54 with the baseof the transistor. With the emitter electrode of the transistor isconnected an envelope forming circuit 44 consisting of a resistor 48 anda capacitor 49 through an emitter resistor 51. A resistor 50 isconnected between the node 55 and one terminal of a key switch 43 ofwhich the other terminal is provided with a potential source V A biasresistor 45 is connected between the nodes 54 and 55. The transmissionof the signal source 41 to the output terminal 53 is controlled by meansof the key switch 43. With such a switching circuit, not only thedesired sustain" effect can be produced, but also a satisfactoryopening-closure ratio can be i obtained and the emitter junctionbreakdown trouble can be i reduced.

, By way of example, the circuit constants of the switching TABLE 2Rectangular wave signal source voltage, V. Diode 52, 131473.

Transistor 42, NPN silicon transistor 2SC458. Resistor 45, 5K9.

Resistor 47, 2009.

Resistor 48, 40KQ.

Resistor 50, 509.

Capacitor 49, 50;:- F.

Potential source V5, 12V.

it will be readily apparent to those skilled in the art that a unipolartransistor such as an insulated gate type field effect transistor may beused in place of a bipolar transistor such as the above NPN transistor,and also that instead of a single PN junction diode use may be made of adiode comprising a plurality of PN junctions connected in series witheach other, a transistor, and another rectifying element in whichconductive and nonconductive regions occur. Furthermore, in theforegoing the signal source has been described and illustrated by way ofexample as a source for providing a rectangular wave signal, it is alsopossible to use a sinusoidal signal source. Also, various modificationsare possible without departing from the scope and spirit of the presentinvention defined in the appended claims.

I claim:

1. A switching device for electronic musical instrument, comprising anamplifier element having a control electrode and a pair of outputelectrodes, an output circuit from which is taken an output signal, saidoutput circuit including said pair of output electrodes, a power sourceand a load impedance, an unsymmetrical type element connected with saidcontrol electrode, a signal source to supply an input signal to saidcontrol electrode through said unsymmetrical type element, said signalsource being connected with said unsymmetrical type element, biasingmeans connected with said unsymmetrical and amplifier elements to imparta first and a second bias conditions thereto so that under the firstbias condition said unsymmetrical type element is biased in the reversedirection and said amplifier element is so biased that its gain becomessubstantially zero while under the second bias condition saidunsymmetrical type element is biased in the forward direction and saidamplifier element is so biased that its gain assumes a predeterminedvalue, switch means for controlling said biasing means to thereby impartthe first or second bias condition to the unsymmetrical type element oramplifier element in accordance with the opening or closing state ofsaid switch means, and a time-constant circuit connected with saidbiasing means and switch means for delaying the switching of the biascondition for a predetermined period of time so that said second biascondition is gradually changed to the first bias condition when saidswitch means is switched fro on" to off".

2. A switching device for electronic musical instrument, comprising atransistor having emitter, base and collector electrodes, an outputcircuit from which is taken an output signal, said output circuitincluding a load impedance, said emitter and collector electrodes and adc. power source, a rectifier element having two terminals and arectifying junction, one of said two terminals being connected with thebase electrode of said transistor so that the conducting direction ofsaid rectifying junction becomes the same as that of the emitter-base PNjunction of said transistor, a signal source connected with the otherterminal of said rectifier element to apply an input signal to said baseelectrode through said rectifier element, means for imparting a firstand a second bias conditions to the connection point between saidrectifier element and said base electrode and the emitter electrode ofsaid transistor so that under the first bias condition said transistoris so biased that its gain becomes substantially zero and saidrectifying junction is biased in the reverse direction while under thesecond bias condition said transistor is so biased that its gain assumesa predetermined value and said rectifyin unction 18 biased in theforward direction, a switch connecte between said emitter electrode andone electrode of said dc. power source for controlling said biasingmeans, said switch being adapted to impart said first or second biascondition to both said connection point and said emitter electrodeaccording to whether said switch is opened or closed, and a CRtimeconstant circuit connected with said biasing means and said switchfor delaying the switching of the bias condition for a predeterminedperiod of time so that said second bias condition is gradually changedto the first bias condition when said switch is switched from on tooff".

3. A switching device for electronic musical instrument, comprising asignal source, a transistor having emitter, base and collectorelectrodes, a PN junction diode connected between the signal source andthe base electrode of said transistor in such a manner that theconducting direction thereof becomes the same as that of theemitter-base PN junction of said transitor, a first potential sourcehaving a predetermined potential relative to a reference potential pointfor operating said transistor, a first resistor connected between saidfirst potential source and the collector electrode of said transistor,an output terminal connected with said collector electrode, a secondresistor connecting the emitter and base electrodes with each other. acapacitor connected between the emitter electrode and the referencepotential point, a key switch connected between the emitter electrodeand the reference potential point, a second potential source having apredetermined potential relative to the reference potential point forimparting bias voltages to said diode and said transistor, the potentialof said second potential source being selected to be higher than thepeak value of the signal voltage, and a third resistor connected betweensaid second potential source and said emitter electrode.

4. A switching device for electronic musical instrument. comprising asignal source, a transistor having emitter, base and collectorelectrodes, a first resistor connected between the collector electrodeand a reference potential point, a PN junction diode connected betweensaid signal source and the base electrode in such a manner that theconducting direction thereof becomes the same as that of theemitter-base PN junction of said transistor, a second resistor connectedbetween the emitter and the base of said transistor, a potential source,a key switch connected between the potential source and the emitterelectrode, a third resistor connected between the emitter electrode andthe reference potential point, and a capacitor connected between theemitter electrode and the potential source.

